The present invention is related to the invention of now abandoned application Ser. No. 229,353, filed Jan. 29, 1981 by the same inventor as herein and assigned to the assignee of the present invention.
The present invention relates to photovoltaic solar cells and specifically to low absorptance solar cells particularly useful in outer space applications.
Silicon solar cells perform most efficiently when operated at low temperatures. Conventional terrestial solar cells when operated in space are excessively heated by solar radiation trapped in the cell, thereby reducing efficacy. Excessive solar radiation trapping in such cells results from cell ineffectiveness in rejecting the unusable portion of the solar spectrum (long wavelength radiation). The fraction of the solar radiation not rejected is called the absorptance. Absorptance should be made as small as possible while not rejecting the useful portion of the spectrum. Textured front cell surfaces have been found to improve the performance of terrestrial solar cells; such surfaces, however, are not suited for space cells due to increased absorptance. High absorptance in conventional textured cells is due to two factors which are largely eliminated by the present invention. First, in such conventional textured cells the front surface is made up of small irregularly shaped pyramids which lead to diffuse scattering and trapping of radiation by total internal reflection. Secondly, a substantial fraction of the surface area of such conventional cells is covered by optically absorbing electrode metallization which causes the trapped radiation to be absorbed before it is able to escape.
The solar cell of the present invention achieves a low operating temperature by rejecting the long-wavelength portion of the solar spectrum. Special optical features of the cell include a silicon wafer with a front radiation-receiving surface selectively etched to produce facets in the (111) crystalline plane which refract the radiation so that it is totally reflected at the wafer back surface. The reflected radiation is returned to the front surface of an angle which allows its escape therethrough. The radiation which does not escape on the first reflection is favored to escape after further reflections by minimizing the fraction of the silicon wafer surface that is contacted by metal electrodes.
The phenomenon of total reflection of light is important to the operation of the invention. The high index of refraction of silicon makes it difficult for light (infrared radiation) which has entered the silicon to escape again. Only light that strikes the silicon surface at nearly normal incidence is able to escape. If the angle of incidence is greater than 17.degree. from perpendicular, light is totally reflected without any absorption loss. This condition is achieved in the present invention by providing faceted areas which refract the light as it enters the silicon such that it is totally reflected from the back surface and directed back to the front surface where it impinges at nearly normal incidence and can then escape. Light that arrives at non-faceted areas of the cell is reflected at the back surface by an oxide layer covered with silver which provides efficient metallic reflection and sends the light back out the front surface again with negligible absorption loss.